A multiplexed EPI pulse sequence can use a simultaneous image (or echo) refocusing (SIR or SER) technique in which multiple RF excitation pulses excite several slices within the pulse sequence. The MR signals for the several simultaneously excited slices are read out in a single echo train Because the RF pulses in this example are spaced by several milliseconds, e.g., 5 ms, there is a corresponding time difference in their signal decay beginning at their respective excitations. This gives a different TE (echo time, of the time the excitation pulse and the peak of the echo signal in EPI), echo time, and image contrast and SNR (signal to noise ratio) in the MR images. A multiplexed EPI technique is described in Feinberg D A, et al., Multiplexed Echo Planar Imaging for Sub-Second Whole Brain FMRI and Fast Diffusion Imaging, PLoS One www.plosone.org, December 2010, Volume 5, Issue 12, e15710, a paper that is incorporated by reference herein and is attached hereto as Appendix A to become a part of this patent specification.
The inherent difference in timing of TE in the SIR images is effectively reduced according to this patent specification by using alternating polarity of the slice selective gradient pulse Gs and corresponding alternate polarity in RF phase offset in the excitation pulse. By using alternating polarity selective gradients, the refocusing gradient pulse on the Gs axis can be eliminated between the excitation pulses so that the time spacing between the multiple SIR excitation pulses is reduced, and therefore the time delay between onset of slice signal decay is reduced. This results in an earlier possible TE of the first excited slice, hence less signal decay and higher SNR, and overall the TE of different slices can be more nearly the same.